This invention relates to the field of small sized electrical resistance heater apparatus of the high radiant energy generating and high operating temperature type--heaters operating in the range of 100 to 200 kilowatts of input power.
As presently envisioned, the military aircraft of the future will include a combination of atmospheric flight capability and above-atmosphere or space flight capability. Inherent in such vehicles is the incurrence of repeated transitions between space and atmospheric flight conditions--transitions which must occur upon exit or reentry of the vehicle into the earth's atmosphere. In presently used rocket-propelled space vehicles it has become common practice to employ high-temperature ceramic or tile materials or other thermal protecting arrangements that are disposed over salient and atmospheric heating prone portions of the flight vehicle for withstanding the high-temperature transition of space to atmospheric flight. In the case of a transatmospheric aircraft, however, it is necessary that wing surfaces and other unshielded portions of the aircraft be capable of withstanding this transition heating without the use of heavy and silhouette complicating additional structures. For such vehicles, the use of carbon-carbon composite materials and actively cooled structures are undergoing intense investigation. This investigation includes testing that simulates conditions expected during vehicle contact with the atmosphere at speeds of Mach 25.
This investigation therefore includes the laboratory testing of vehicle related structures at temperatures in the 3000.degree.-4500.degree. F. range.
In addition to these aircraft-spacecraft vehicle testing needs, there is a presently well-developed effort attempting to improve the heat resistance characteristics of artillery and rocket nose cone components--components which also involve the use of such arrangements as carbon and laminated carbon structures. In the testing of a nose cone tip portion for erosion characteristics, for example, it is desirable to subject the nose cone to both elevated temperatures and other hostile environment conditions in order to assess its accuracy retaining capabilities.
In addition to all of these flight vehicle related activities, there is an ongoing need for high-temperature generating apparatus in a plurality of industrial and commercial arts that are historic and evolving in nature. Included in these activities is fabrication and testing of the ceramic materials used in glass and steel melting furnaces and in the fabrication of high-temperature industrial apparatus such as the ceramic insulators and heaters used in generating high-temperature environments.
The use of electrical resistance heating provides a number of significant advantages in each of these high-temperature environments, advantages which overshadow the limited choice of resistance materials capable of operating at such temperatures. Previous resistance heating test apparatuses have included the use of metallic elements and glass or quartz enclosed heating sources, i.e., high-temperature quartz lamps, and also the use of graphite inclusive heating elements in a variety of configurations. In practice however, it is found that metallic elemented, quartz enclosed heaters are limited in temperature generation and heat flux rates to values in the range of 2700.degree. F. and 140 BTU per square foot second. Contemplated test needs in the transatmospheric vehicle field, however, include temperatures in excess of 4,000.degree. F. and heat flux rates exceeding 300 BTU per square foot second. Clearly such quantities are beyond the capability of the quartz enclosed metallic filament heat source.
The heater disclosed in U.S. Pat. No. 3,573,429 of F. W. Brodbeck et al is an example of a graphite element based heating apparatus that is capable of operating in the temperature and heat flux ranges needed for transatmospheric vehicle testing. Graphite heating elements are in fact capable of producing heat flux densities in the range of 400 BTU square foot second.
The heater described in the Brodbeck patent, notwithstanding these underlying capabilities, however, has been found to be limited by a number of practical considerations, considerations which include high initial and operating costs, unpredictable physical integrity--particularly when operated in inverted positions, inability to maintain satisfactory electrical connections, difficulty in establishing and maintaining cooling system closure, and thermal-physical stress induced short operating life.
Other examples of graphite heating apparatuses are to be found in the patents of D. M. Harris, U.S. Pat. No. 3,351,742, and H. G. Wilsey, U.S. Pat. No. 3,755,658. The Harris patent is concerned with a graphite heating element in which the cross-sectional area is varied throughout the element length in order to maintain a desirable element temperature profile. This desired cross-sectional variation is illustrated to be achieved in a plurality of manners in the Harris invention and is used in order that the temperature uniformity requirements of an integrated circuit and epitaxial deposition furnace be met. It is also notable that the energy densities and operating temperatures recited in the Harris patent are significantly below those of the present invention.
In the Wilsey patent is disclosed a graphite heater which has a circular or cylindrical configuration and a heat generating path that is defined by a plurality of alternating upward and downward facing zig-zag picket members. A plurality of circumferential slot cuts serve to for the zig-zag picket members comprising the Wilsey heating element. The Wilsey apparatus is especially suited for use in crystal growing furnaces and similar lower temperature apparatus. The use of graphite screws for joining the heating element to a graphite power connector element is a notable aspect of the Wilsey heater.
As indicated by these patent examples, numerous arrangements of graphite elements are known in the heating art. Notwithstanding these examples however, there has been a notable lack of an operationally satisfactory low-cost graphite heater that is capable of generating large value heat flux in the 4500.degree. F. or 2500.degree. C. range of temperatures with practical convenience.